Etch stop for use in etching of silicon oxide

ABSTRACT

A etch stop layer for use in a silicon oxide dry fluorine etch process is made of silicon nitride with hydrogen incorporated in it either in the form of N—H bonds, Si—H bonds, or entrapped free hydrogen. The etch stop layer is made by either increasing the NH 3  flow, decreasing the SiH 4  flow, decreasing the nitrogen flow, or all three, in a standard PECVD silicon nitride fabrication process. The etch stop can alternatively be made by pulsing the RF field in either a PECVD process or an LPCVD process.

This application is a Continuation of application Ser. No. 09/370,080,filed on Aug. 6, 1999, now U.S. Pat. No. 6,222,257, which is aContinuation of application Ser. No. 08/850,461, filed on May 5, 1997,which issued on Jan. 11, 2000 as U.S. Pat. No. 6,013,943, which is aDivisional of application Ser. No. 08/558,777, filed on Nov. 15, 1995,which issued on Dec. 21, 1999 as U.S. Pat. No. 6,004,875.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention in general relates to integrated circuit fabricationprocesses and more particularly to a silicon-nitride-based material foruse as an etch stop when etching silicon dioxide in such fabricationprocesses and a method of making such an etch stop material.

2. Statement of the Problem

Integrated circuits are mass produced by fabricating hundreds ofidentical circuit patterns on a single semiconductor wafer.Subsequently, the wafer is sawed into hundreds of identical dice orchips. While sometimes referred to as “semiconductor devices”,integrated circuits are in fact fabricated from patterned layers ofconductors, insulators, and semiconductors. Silicon, the most commonlyused semiconductor material, is used in either the single crystal orpolycrystalline form. One type of semiconductor wafer fabricationprocess, which the embodiment of the invention described hereinutilizes, begins with a single crystal silicon substrate. Silicondioxide is also commonly used in integrated circuits as an insulator ordielectric. Its use is so common that in the art is generally referredto as “oxide” without ambiguity.

Silicon dioxide is routinely used in integrated circuit fabricationtechnology by forming a layer of the material on the integrated circuitwafer, then removing portions of the layer in a photo mask and etchprocess. In such a photo mask and etch process, a photo mask containingthe pattern of the structure to be fabricated is created, then, afterformation of the silicon dioxide layer, the wafer is coated with alight-sensitive material called photoresist or resist. Resists aretermed either positive or negative, a negative resist being a materialwhich on exposure to light becomes less soluble in a developer solution,and a positive resist being a material which on exposure to lightbecomes more soluble in a developer solution.

The resist-coated wafer is then exposed to ultraviolet light through themask. Upon exposure to the light, cross polymerization occurs in anegative resist making it insoluble in organic-based developers; in apositive resist, carboxylic acid groups are formed, rendering it solublein basic pH media. The more soluble parts of the resist are then removedin a process called “developing.” The wafer is etched to remove thesilicon oxide unprotected by the resist, and then the remaining resistis stripped. This masking process permits specific areas of the silicondioxide to be formed to meet the device design requirements.

In the etch process described above, it is important that the etchselectively remove the unwanted silicon dioxide and that the materialunderlying the silicon dioxide layer is not damaged. A common way toaccomplish this is to deposit or otherwise form an etch stop layer onthe wafer prior to formation of the silicon dioxide. Such etch stoplayers are commonly made of an insulating material that is resistant tothe particular etch process used. In the integrated circuit fabricationart, the property of being resistant to an etch process is called the“selectivity” of a material. The selectivity S of a particular materialin a particular etch process is usually defined as the etch rate of thematerial to be removed divided by the etch rate of the particularmaterial. Thus, a material that is highly resistant to an etch is saidto have a high selectivity. Common insulators that have a highselectivity in silicon dioxide etch processes and are used as etch stoplayers are aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), and siliconnitride (Si₃N₄). Aluminum oxide and titanium dioxide present a risk ofcontamination of the wafer with metal and other particles that cancreate defects in the electrical circuits, and thus are used sparingly,and in particular are generally not used in high volume integratedcircuit manufacturing processes such as DRAM fabrication. Siliconnitride has been used as an etch stop layer for etching silicon dioxidein DRAM fabrication processes.

One common method of etching silicon dioxide is in a fluorine plasma,such as CF₄, C₂F₆, SF₆, or NF₃, while using silicon nitride as an etchstop. Often fluorine plasma etches are performed under conditions thatprovide a directional or anisotropic etch so that the sidewalls of theetched feature are nearly vertical. It is known to add polymerizingagents, such as C₂F₂, CHF₃, and other such carbon containing materialsto a fluorine plasma to provide a high selectivity to silicon nitride.It has also been found that if enough hydrogen is present in the plasma,such polymerizing agents can condense out on the silicon dioxide andstop the etch of the silicon dioxide. If an oxygen source is supplied tothe plasma, the deposition of the polymers decreases or is preventedaltogether. However, increased oxygen tends to make the etch lessdirectional or more isotropic.

There is much literature in the art related to fine tuning such fluorineetches by use of polymers and oxygen to maximize the selectivity tosilicon nitride while preventing polymer deposition. All of this artteaches that adding large quantities of hydrogen to the plasma iscounterproductive, due to the deposition of polymer on the reactor wallsand narrow process windows. Further, the art relating to the use ofsilicon nitride in integrated circuit manufacturing processes teachesthat hydrogen should be excluded because it lightens the material, whichis considered to be detrimental to a dielectric. The plasma etch artshows also that it is very difficult to control the oxide etchingprocess described above: that is, the better the selectivity, the morechance there is of polymer deposition.

These processes are thus balanced on the edge of disaster, with thestrong possibility of sudden, massive depositions of polymer that ruinthe wafers. There is a need therefore for a silicon dioxide etchingprocess and etch stop layer material that provides high selectivitywhile also being controllable, without the potential of contamination ofthe wafer.

Generally, a change in one phase of the integrated fabrication processimpacts other phases. Since integrated circuit fabrication processes arehighly complex and require sophisticated equipment, developments ofentirely new processes and materials can be quite costly. Thus newmethods and materials for silicon dioxide etching that can beincorporated into current fabrication technology would be desirablebecause expensive modification of equipment and processes can beavoided.

3. Solution to the Problem

The present invention solves the above problems by providing an etchstop material with improved selectivity. The invention provides one ormore methods of making silicon nitride etch stop material that hashigher selectivity in silicon dioxide etch processes than siliconnitride made by prior art processes.

Analysis indicates that the invention provides an etch stop materialcomprising silicon nitride containing added hydrogen. It has been foundthat such a hydrogen-enriched silicon nitride has an increasedselectivity in anisotropic or directional silicon dioxide etchprocesses, without creating wafer contamination problems that causedefective integrated circuits.

SUMMARY OF THE INVENTION

The invention provides a method of fabricating an integrated circuitcomprising: providing a semiconductor wafer; creating a layer of siliconnitride on the wafer incorporating hydrogen in the silicon nitride;creating a silicon oxide layer on the layer of silicon nitride; andpatterning the silicon oxide in a process including directionallyetching through selected areas of the silicon oxide and stopping theetch on the silicon nitride layer. The present invention provides aSi₃N₄ film with elevated hydrogen content to interact with a dry etchprocess to retard the etch rate. Any method used to increase hydrogencontent in a Si_(y)N_(x) film may be employed, including ionimplantation, optimization of PECVD or LPCVD, and other processes wherethe hydrogen content of the film is elevated above that in the standardprior art process.

The invention also provides an integrated circuit comprising a layer ofsilicon dioxide and an etch stop, the etch stop means comprising siliconnitride incorporating hydrogen. The invention not only provides anenhanced etch stop layer for use in etching silicon dioxide, but offersone that can be made with standard integrated circuit fabricationapparatus and processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a semiconductor waferin accordance with the present invention just prior to the silicondioxide etch;

FIG. 2 is a cross-sectional view of the wafer of FIG. 1 after the oxideetch and resist strip;

FIG. 3 is an H₂ depth profile comparison of seven semiconductor waferillustrating the entrapped H₂ in wafers made by various processes;

FIG. 4 is an N—H depth profile comparison of seven semiconductor waferillustrating the N—H bonded hydrogen in wafers made by variousprocesses;

FIG. 5 is an O—H depth profile comparison of seven semiconductor waferillustrating the O—H bonded hydrogen in wafers made by variousprocesses; and

FIG. 6 is a depth profile comparison of single semiconductor waferprocessed by PECVD in a pulsed RF field.

DESCRIPTION OF THE PREFERRED EMBODIMENT 1. Overview

FIG. 1 shows a cross-sectional view of a semiconductor wafer 10according to the preferred embodiment of the invention. It should beunderstood that the figures are not meant to be actual cross-sectionalviews of any particular portion of an actual semiconductor device, butare merely idealized representations which are employed to more clearlyand fully depict the process of the invention than would otherwise bepossible.

Wafer 10 comprises a semiconductor substrate 12, preferably, lightlydoped P-type silicon, silicon nitride etch stop layer 14, silicondioxide layer 16, and resist layer 18. Resist layer 18 has beensubjected to a photo mask and etch process to define an area 19 wherethe silicon dioxide 16 is to be patterned by a directional oxide etch.In an actual semiconductor wafer 10, the silicon substrate may be morecomplex than shown, including doped wells and other structures which arenot pertinent to the invention and therefore are not shown. Othernon-pertinent structures may also be present.

The term “silicon nitride” herein means Si_(x)N_(y), where x isgenerally 3 but may also be 4 or any other number which indicates anumber of atoms of silicon that will form a stable compound with ynitrogen atoms, and y is usually 4 but may be any other number thatrepresents a number of nitrogen atoms that will form a stable compoundwith x silicon atoms. The term “hydrogenated silicon nitride” as usedherein means silicon nitride having atomic hydrogen and/or hydrogencompounds incorporated therein in sufficient concentration so as toaffect etch rate of the hydrogenated silicon nitride as compared toconventional silicon nitride.

In FIG. 2 the directional oxide etch, preferably a dry etch usingfluorine chemistry, has been performed. Oxide 16 has beenanisotropically etched through in area 22 to expose a portion of siliconnitride etch stop layer 14. In practice, the oxide etch may penetratethe silicon nitride layer 14 a small amount. Thus, when it is statedthat the etch stops “on” the silicon nitride, the word “on” includesboth the case where the etch goes to the silicon nitride 14 and stopsand the case where it also to some degree etches the silicon nitride andstops in the silicon nitride. Although silicon nitride etch stop layer14 is illustrated in a position below or underneath etchable oxide layer16, it is expressly understood that etch stop layer 14 will function inany position (i.e., above, below, or along side) an etchable oxidelayer. Etch stop layer 14 need only be adjacent to etchable oxide layer16 to meet the needs of the present invention.

In accordance with the preferred embodiment, silicon nitride layer 14has been formed by one of a group of novel processes that producesilicon nitride with enhanced selectivity in the oxide etch. Analysis,which shall be discussed in detail below, indicates that the group ofprocesses increase the hydrogen content of the silicon nitride 14preferably by incorporating hydrogen in quantities greater than 1×10¹⁵atoms per cubic centimeter utilizing N—H bonding, Si—H bonding, or freehydrogen entrapment. Hence, silicon nitride 14 when manufactured inaccordance with the present invention is alternatively referred to ashydrogenated silicon nitride. Increased hydrogen content is believed tobe the reason for the enhanced selectivity.

2. Detailed Description of Fabrication Process

Returning now to FIGS. 1 and 2, the processes of forming substrate 12,oxide 16, and resist 18 layers are conventional and will not bediscussed in detail herein. The photo mask and etch process and theoxide etch that patterns silicon oxide 16 are also processes that arewell-known in the art and thus will not be discussed in detail herein.The preferred oxide etch is an anisotropic dry fluorine-chemistry basedetch. As is well-known, such etches utilize a plasma of gases, such asCF₄, C₂F₆, SF₆, or NF₃, that in a plasma provide fluorine radicals whichact as chemical etchants, and the plasma also provides ions that areaccelerated toward the surface of the silicon oxide 16 and etch it byforming volatile compounds. The process of forming the silicon nitridelayer 14 may be any one of a group of variations of conventional siliconnitride formation processes.

The preferred process for formation of the silicon nitride 14 is a PECVD(Plasma Enhanced Chemical Vapor Deposition) process utilizing an AppliedMaterials Deposition tool, model Precision 5000, known as an AMD 5000™in the art. A series of wafers of silicon nitride deposited on a siliconsubstrate were made in the AMD 5000™ PECVD process utilizing thefollowing basic recipe: 2000 sccm N2, 55 sccm NH3, 125 sccm SiH4, RFPower 350 Watts, susceptor set point temperature 400° C. Variousalterations of this basic recipe were also performed, namely varying theflow rates of the supply gases. The films were subsequently etched, alsoin an Applied Materials Precision 5000 Etcher, known as an AME 5000 inthe art, using a single etch condition, namely 35 sccm CHF3, 25 sccmCF4, 60 sccm Ar, 700W RF Power, and a magnetic field of 75 Gauss.

TABLE I SiH₄ NH₃ N₂ ETCH WAFER FLOW FLOW FLOW RATE NO. (SCCM) (SCCM)(SCCM) (A/SEC) 1 200 55 2000 37.13 2 200 55  500 30.35 3 200 90 125033.44 4 200 125  2000 36.62 5 200 125   500 34.60 6 130 55 1250 31.78 7130 90 1250 30.64 8 130 90  500 32.31 9 130 90 2000 26.64 10   55 552000 17.71 11   55 55  500 20.24 12   55 90 1250 18.90 13   55 125  200017.26 14   55 125   500 17.84

The etch rate of the samples when etched in a dry fluorine-based oxideetch was measured with the results shown in Table I. As seen in Table I,reducing the flow of silane, increasing the flow of ammonia, anddecreasing the flow of nitrogen all yield lower etch rates. In a fewinstances reducing the nitrogen flow rate when the silane flow rate hadalready been reduced or the ammonia flow rate had already been raisedincreased the etch rate.

With the above evidence that the selectivity of the silicon nitridecould be increased dramatically by decreasing the flow of silane,increasing the flow of ammonia, and decreasing the flow of nitrogen inthe conventional silicon nitride formation process, seven siliconsamples were made with the intention of discovering the reason for theincreased selectivity. Wafer No. A1 was a control sample made by thestandard process for making 1000 A thick silicon nitride in an AMD 5000™PECVD which is described in Table II. The SCCM units used in Table IIare standard cubic centimeters per minute. The processes used to makeseven sample wafers are summarized in Table III.

TABLE II PREDEPOSITION DEPOSITION STEP STEP DEPOSITION 5.0 9.7 SECONDSTIME SECONDS HELIUM 4.5 4.5 TORR PRESSURE TORR SUSCEPTOR 400° C. 400° C.SET POINT WAFER 349° C. 361° C. TEMPERATURE POWER 50 WATTS 350 WATTSNITROGEN FLOW 2000 SCCM 2000 SCCM AMMONIA FLOW 55 SCCM 55 SCCM SILANEFLOW 0 125 SCCM

TABLE III WAFER FABRICATION THICK NO. PROCESS SUMMARY NESS A1 AMD 5000 ™CONTROL 1000A A2 2 × NH₃ 1000A A3 2 × NH₃, ½ N₂ 1000A A4 2 × NH₃, ½ N₂,¼ SiH₄ 1680A A5 2 × NH₃, 1\2 N₂, .78 PRES. 1000A A6 STANDARD LPCVDPROCESS 2400A A7 STAND. NOVELLUS ™ PECVD, PULSED 5400A

In Table III the numbers indicated the amount that the flow of theparticular gas was increased or decreased as compared to the AMD 5000™control process; for example, 2×NH₃ indicates that the flow of ammoniawas doubled to 110 SCCM. For all the wafers the silicon nitride wasdeposited on a single crystal silicon substrate.

Three of the above samples (A4, A6, and A7) were wet etched with an HFsolution to give a quick and rough idea of hydrogen content. The resultsare shown in Table IV. In Table IV the etch concentration gives theratio of deionized water to hydrofluoric acid (HF) used in the etch;that is, an etch concentration of 5:1 indicates that the hydrofluoricacid was diluted with 5 parts of deionized water to 1 part of HF. In thewet etch process, the hydrogen in the silicon nitride film increases theetch rate in an HF containing solution and therefore may be used as anindicator of hydrogen content.

TABLE IV WAFER NO. ETCH CONC. ETCH RATE A4 5:1 37.3 A/SEC A6 5:1  0.9A/SEC A7 5:1 19.6 A/SEC

The results of Table IV suggest that wafer A6 has the lowest amount ofhydrogen and wafer 4 contains the highest amount of hydrogen. The sevensample wafers were then analyzed for hydrogen content by performing SIMS(Secondary Ion Mass Spectroscopy) profiles with the results summarizedin FIGS. 3-6. The SIMS profiles shown in FIG. 3-FIG. 6 are presented toshow qualitative effects of the various processing and are notnecessarily quantitatively accurate because of film thicknessdifferences between the samples and other random noise generated duringthe SIMS analysis.

In the SIMS set up the sputtering ion beam was 7 keV cesium atoms withthe following characteristics: measured current of 30 nanoamps, incidentraster of 500 mm², angle of incidence=60 degrees with respect toanalyzer (note: analyzer is normal to the surface of the sample),primary gating 70%, the mass resolution was 30, the sputter rate was 100A/minute, and the relative scanning factor used for all curves was1×10²⁰ with respect to a silicon isotope with an atomic mass of 56units.

FIGS. 3 through 5 confirm that wafer A4 has the highest hydrogenconcentration while wafer A6 has the least. The others fallappropriately in-between to indicate that the increased selectivity isdue to hydrogen content. In FIG. 6 the seven peaks of increased hydrogencontent correspond to seven pulses off and on of the RF filed during thePECVD fabrication process. In other words, each time the RF filed wasturned off then turned back on, a peak appears in the hydrogen content.This shows that pulsing the RF field during a PECVD process or itsequivalent increases the hydrogen content.

It is postulated that the increased hydrogen increases the selectivityby combining with carbon and halogens that are in the etch plasma toform halocarbon polymer deposits on the surface of the silicon nitride,which deposits slow or stop the etch process. The carbon may beintroduced into the plasma either in the feed gas, from the etch mask,or from contaminants in the system. Further, N—H bonds have thepotential of reacting with fluorine to form nonvolatile ammoniumfluoride at the silicon nitride surface, which can further impede orstop the etching of the nitride film.

From the above, it is clear that any manner of adding the hydrogen tothe silicon nitride will be effective in providing an enhanced etch stoplayer 14. For example, the hydrogen content of the silicon nitride maybe enhanced by any one of a number of relatively simple adjustments tothe standard silicon nitride deposition processes: e.g. hydrogen gas maybe added to the standard CVD or PECVD process, which will increase thehydrogen content of the silicon nitride. Another manner of depositingthe silicon nitride that can provide better control of the materialsbeing deposited is ECR (Electron Cyclotron Resonance) deposition.Preferably, silicon nitride etch stop in accordance with the presentinvention incorporates hydrogen in quantities greater than approximately1×10¹⁵ atoms per cubic centimeter utilizing N—H bonding, Si—H bonding,or free hydrogen entrapment based upon the concentrations at which it isbelieved to begin to depress the etch rate of the silicon nitride.

It is also evident that the etch stop layer provided by the inventionwill be an effective etch stop for etches other than a silicon dioxideetch. For example, the layer 16 may be made of a low-hydrogen-contentSi_(x)N_(y), where x is any number of silicon atoms which will combinewith nitrogen, and y is any number of nitrogen atoms that will combinewith x silicon atoms. Likewise, as indicated above, the layer 14 mayalso be made of a silicon nitride other than Si₃N₄. It may be made ofSi_(x)N_(y), where x and y have the same meaning as indicated above, aslong as the material that is to be the etch stop layer 14 is hydrogenenriched.

There has been described a novel etch stop material which has higherselectivity in a dry, fluorine-based silicon dioxide etch and which hasmany other advantages. A number of novel processes for making thematerial have also been described. It should be understood that theparticular embodiments shown in the drawings and described within thisspecification are for purposes of example and should not be construed tolimit the invention which will be described in the claims below.Further, it is evident that those skilled in the art may now makenumerous uses and modifications of the specific embodiments described,without departing from the inventive concepts. For example, now that itis seen that hydrogen enhances the etch stop qualities of siliconnitride, many other ways of adding hydrogen to the material may bedevised. Or the various processes described to make the material may bevaried greatly. Or the hydrogen-enriched material may be used as an etchwith other etch processes. Consequently, the invention is to beconstrued as embracing each and every novel feature and novelcombination of features present in and/or possessed by the semiconductordevice described.

What is claimed is:
 1. An integrated circuit comprising: a substrate; alayer of hydrogenated silicon nitride disposed on the substrate; and anetched layer of silicon dioxide disposed over the layer of hydrogenatedsilicon nitride, selected areas of the etched layer being removedstopping on the layer of hydrogenated silicon nitride.
 2. The integratedcircuit, as set forth in claim 1, wherein the layer of hydrogenatedsilicon nitride comprises N—H bonds.
 3. The integrated circuit, as setforth in claim 1, wherein the layer of hydrogenated silicon nitridecomprises entrapped hydrogen.
 4. The integrated circuit, as set forth inclaim 1, wherein the layer of hydrogenated silicon nitride has a firstselectivity to a given etchant and the etched layer has a secondselectivity to the given etchant, the first selectivity being higherthan the second selectivity.